In modern internal combustion engines, devices for variably adjusting the timing control of gas exchange valves are used in order to configure the phase relation between a crankshaft and a camshaft variably between a maximum advance and a maximum retard position, within a defined angular range. The device comprises a hydraulic phase adjustment device and a control valve. The phase adjustment device comprises a drive element, an output element and at least one pair of counter-working pressure chambers. The drive element is in driving connection to the crankshaft, for example by means of a traction drive or gear drive. The output element is in driving connection to the camshaft, which connection usually is effected by a non-rotatable attachment of the output element to the camshaft. By means of appropriate charging of the pressure chambers with pressure medium and discharging thereof, the phase relation of the output element relative to the drive element (and therefore of the camshaft relative to the crankshaft) can be adjusted variably within a defined angular interval. Phase adjustment devices of this type are embodied, for example, as vane cell adjusters or axial piston adjusters, which are known from DE 101 50 856 A1 and DE 42 18 082 A1 and to the disclosed content of which reference is made herewith.
The flows of pressure medium from and to the pressure chambers are regulated by means of the control valve. The control valve generally comprises a hollow-cylindrical valve housing on which are embodied an inflow port, an outflow port and two working ports. The inflow port is connected to a pressure medium pump of the internal combustion engine and the outflow port to a tank. Each of the working ports communicates with one of the pressure chambers.
The control valve, generally in the form of a proportional control valve, has a control piston arranged to be axially displaceable within the valve housing, by means of which control piston the working ports can be connected selectively to the inflow port or the outflow port. Thus, pressure medium can be supplied to the first pressure chamber while at the same time pressure medium can be discharged to the tank from the second pressure chamber, or vice versa. The flows of pressure medium from and to the phase adjustment device can therefore be regulated by means of the control valve.
The pressure medium is usually made available by a pressure medium pump of the internal combustion engine. Under unfavorable operating conditions, for example at low rotational speeds of the internal combustion engine with high pressure medium temperatures, the volume flow delivered by the pressure medium pump, or the operating pressure resulting therefrom, may be insufficient to execute the phase adjustment specified by an engine control device at the desired speed. Furthermore, during the switch-off process of the internal combustion engine, the operating pressure may be insufficient to set the phase relation to a base position in which unproblematic restarting of the internal combustion engine is possible. To eliminate these problems, it is known to provide auxiliary pressure sources, for example pressure accumulators, which make available an additional volume of pressure medium in order to make possible the desired adjustment in case of an insufficient quantity of pressure medium delivered by the pump.
A device of this type is known, for example, from EP 0 806 550 A1. In this embodiment, a pressure accumulator is provided as the auxiliary pressure source and communicates with a pressure medium line which connects the pressure medium pump to the inflow port of the control valve. A disadvantage of this embodiment is that the pressure within the pressure accumulator corresponds to the operating pressure delivered by the pressure medium pump at all times. The volume stored in the pressure accumulator therefore decreases when the operating pressure falls. Pressure medium is therefore already supplied from the pressure accumulator to the pressure medium system of the internal combustion engine when the operating pressure is falling but has not yet fallen below the critical value. In the critical operating phases of the internal combustion engine, therefore, the total volume of pressure medium which can be stored in the pressure accumulator is not available in the phase adjustment device.
A further device is known from U.S. Pat. No. 5,704,317 A. In this embodiment the pressure accumulator also communicates with a pressure medium line which connects the pressure medium pump to the control valve. To prevent a premature outflow of pressure medium from the pressure accumulator, a directional control valve which, in a first control position, allows filling of the pressure accumulator, is arranged between the pressure medium line and the pressure accumulator. In a second control position, the pressure accumulator is connected to the inflow port of the control valve. A disadvantage of this embodiment is the large space requirement entailed by the two separate control valves.